"Filemaker cannot share files because another user is already sharing files using Filemaker Pro on this computer"

This problem only seems to be related to Windows Vista.

Follow these steps which should rectify the problem

Windows Vista and Windows 7 UsersClick on Start (windows start menu)
Click on Network
Click on Network and Sharing Center (along the top)
Next to your network connections there should be a View Status button.
Click on View Status
Click on Properties
A list will load up with various items
Locate and uncheck ' Internet Protocol Version 6 (TCP/IPv6)
(DO NOT CONFUSE WITH V4 of the same)
Click Ok
Restart your PC.